This study investigated the interplay of working memory capacity and language proficiency in the context of vocabulary acquisition through word-focused tasks. The involvement load hypothesis served as a theoretical framework, positing that the degree of cognitive engagement influences lexical learning outcomes. A total of 100 EFL learners participated in this study. They were divided into distinct groups based on varying levels of working memory capacity, language proficiency, and aspects of working memory (phonological short-term memory and executive working memory). The participants engaged in receptive and productive vocabulary tasks under different experimental conditions, including reading-only, reading plus blank-fill, and reading-plus production. The results revealed intricate relationships between working memory capacity, language proficiency, and vocabulary acquisition. While the efficacy of the hypothesis varied across conditions, its predictions were influenced by the nuances of individual cognitive capabilities and language competence. Learners with higher proficiency levels acquired more lexical items, both receptively and productively. Although the difference with the phonological short-term memory was not significant, executive working memory was more facilitative of lexical gain. Additionally, the role of input modality in shaping vocabulary learning outcomes was highlighted. That is, reading plus production and blank-fill tasks resulted in more lexical gain than reading-only tasks. The study contributes to the theoretical understanding of vocabulary acquisition by underscoring the complex interplay of cognitive processes and language factors. These insights hold implications for foreign language pedagogy, guiding educators in crafting more effective interventions for enhancing both receptive and productive lexical knowledge.
